The present invention relates to a vehicle seat belt pretensioner and, in particular, to a pretensioner of the type that pretensions the belt by rotating the take-up shaft of a belt retractor.
In some vehicle seat belt systems, a pretensioner is provided for tightening the belt so that it firmly engages the occupant in a collision before the occupant is thrown forward. One type of seat belt pretensioner is disclosed in the Japanese Patent Laid-Open Publication No. 1-119454.
In that pretensioner, a pulley is connected to a take-up shaft of a seat belt retractor through a coupling device, and the pulley is rotated by a cable, which is wound around the pulley and is pulled by means of a drive unit (power source). The coupling device comprises a pair of gears, one of which is fixed on the retractor take-up shaft and the other of which is fixed on the pulley. The pulley is supported on a guide unit in the form of a pivoting arm. In the set position, the guide unit is maintained by a shear pin in a position in which the gears are not engaged. When the drive unit is operated, the shear pin is broken stepwise, first to allow the guide unit to pivot and then to allow the pulley to rotate. After the two gears are engaged with each other by swinging of the guide unit, the pulley is rotated and imparts rotation to the take-up shaft so as to wind a length of the belt onto the belt reel.
In the unit described above, the rotation of the pulley gear must be restricted before the two gears are engaged with each other, and this requires a high-precision holding device (i.e., a shear pin with accurate shear load) operating in two steps. Also, because the device provides high power instantaneously, in the order of milliseconds, even slight resistance may exert a strong influence on the operation of the unit. Because the pulley is supported by the guide device, a sturdy guide device matching the applied power is required, and a high power loss occurs in moving the guide device.